A full psychrometric chart that does the work for you. Plot any air state from two known properties and read off all ten, model heating, cooling, dehumidification, humidification and mixing processes, or enter probe readings from a live system and get a plain English diagnosis. Built on ASHRAE Fundamentals correlations, with metric and imperial units, altitude correction and a branded PDF export.

What a psychrometric chart shows

Air is never just a temperature. The same 21 degrees feels, behaves and condenses completely differently at 30 percent relative humidity than at 70 percent, and every air conditioning, ventilation and drying process is really a journey across two dimensions: how warm the air is and how much water it carries. The psychrometric chart maps that territory. One point on the chart pins down everything about an air state: dry bulb and wet bulb temperature, relative humidity, moisture content, dew point, enthalpy, specific volume and density.

Traditionally that meant a paper chart, a ruler and practice. This tool does the plotting and reading for you: enter any two properties, for example dry bulb and relative humidity from a thermo hygrometer, or dry bulb and wet bulb from a sling, and it calculates and plots the full state instantly. Choose the classic ASHRAE projection or a simpler rectangular view, and toggle the curve families (RH, wet bulb, enthalpy, specific volume) on and off to keep the chart readable.

Three modes, three jobs

State point calculator. The core chart. Two known properties in, ten properties out, plotted live. Includes worked examples to learn from, altitude correction with a UK postcode lookup that fetches your site elevation automatically, and metric or imperial units throughout.

Process builder. Model what the plant actually does to the air:

• Sensible heating and cooling: temperature change with constant moisture
• Cooling with dehumidification: the real behaviour of a cooling coil, including the apparatus dew point and coil bypass factor
• Humidification: steam and evaporative, which take visibly different paths across the chart
• Mixing of two airstreams: fresh and return air combined on a true enthalpy weighted mixing line, not a simple temperature average

Each process reports the load, the moisture added or removed and the resulting state, drawn on the chart so the whole story is visible at a glance.

Installer diagnostic mode. Enter return and supply air readings from a running system, the airflow and optionally the unit's rated capacity, and the tool calculates the actual sensible, latent and total duty the coil is delivering and tells you in plain English what the numbers suggest: whether the symptoms point to an airflow restriction, such as a blocked filter or crushed flexi, or to a refrigerant or coil problem. It is the difference between guessing and knowing before the gauges come out.

Methods and accuracy

All properties are computed from published ASHRAE Handbook Fundamentals correlations, including the Hyland Wexler formulation for saturation pressure, with barometric pressure corrected for altitude. The engine has been verified against ASHRAE worked examples. Everything runs in your browser: nothing you enter is sent anywhere.

Results are indicative and intended for design assistance, education and first line diagnosis. Equipment selections and formal designs should be confirmed by a competent engineer, and refrigerant work must only be carried out by F-Gas certified engineers.

Who this tool is for

Installers diagnosing poor performance from supply and return readings instead of guesswork. Engineers checking coil processes, mixing conditions and room loads without digging out the paper chart. Designers and students learning how conditioning processes actually move air around the chart, with worked examples built in. Every plotted chart and diagnosis exports to a branded PDF for job records or client reports.

Frequently asked questions

What is a psychrometric chart used for? A psychrometric chart maps the relationship between air temperature and moisture content. HVAC engineers use it to find air properties, size cooling and heating coils, design humidification and check whether surfaces will hit dew point and condense. This interactive version does the plotting and calculation automatically.

Is it psychrometric or psychometric? Psychrometric, from the Greek psychros meaning cold, is the correct term for the study of moist air. Psychometric, with no r, refers to psychological measurement, but the misspelling is so common in the HVAC trade that both terms are widely searched. Either way, this is the chart you were looking for.

How do I read a psychrometric chart? Dry bulb temperature runs along the horizontal axis and moisture content up the vertical axis. Curved lines show relative humidity, diagonal lines show wet bulb temperature and enthalpy. Where any two known property lines cross is your air state, and every other property can be read from that point. This tool finds the intersection for you from any two inputs.

What is the difference between wet bulb and dry bulb temperature? Dry bulb is ordinary air temperature measured by a standard thermometer. Wet bulb is measured with a moistened sensor and is lower because evaporation cools it; the drier the air, the bigger the gap. Together they fix the air state completely, which is why the pair is a classic field measurement.

How do I know if my air conditioning has low refrigerant or poor airflow? The supply and return air conditions tell different stories. A restricted airflow produces a large temperature drop across the coil at reduced duty, while a refrigerant or coil fault typically shows a small temperature drop and low delivered capacity. Enter your probe readings in the diagnostic mode above and the tool works through the logic for you.

What is the apparatus dew point? The apparatus dew point (ADP) is the effective coil surface condition a cooling and dehumidifying process aims towards on the chart. Together with the bypass factor, the fraction of air that passes through the coil unchanged, it describes how a real coil performs. The process builder calculates and plots both.

Related tools

• [Cooling and Heating Load Estimator]: establish the duty before analysing the process
• [Refrigerant Pipe Sizing Calculator]: when the diagnosis points to the refrigerant circuit
• [Duct Sizing Calculator]: when the diagnosis points to airflow restriction
• [Part F Ventilation Calculator]: fresh air rates for the mixing calculations